Conventionally, an impact absorber having a hollow wall structure, which is formed by blow molding of thermoplastic resin, as disclosed in, for example, Patent Document 1 has been known.
The impact absorber as disclosed in Patent Document 1 is provided for protecting, for example, the low back of a passenger and has a first portion corresponding to the pubis of the passenger and a second portion corresponding to the ilium of the passenger. The first portion is made such that the stress occurring when the impact absorber is subjected to impact is a predetermined value or below, and the second portion is made such that the stress occurring when the impact absorber is subjected to impact is a predetermined value or above.
In a specific embodiment disclosed in Patent Document 1, the first portion of the impact absorber may be relatively great and the second portion of the impact absorber may be relatively little in terms of the total thickness in a direction where the impact is applied (i.e., the direction of impact). A concave rib is formed by recessing the walls which are opposed to each other in the direction of impact and joining tips of recessed walls as thus obtained each other. In this case, as due to the load from the passenger the impact absorber is deformed with compression, the concave rib renders the stress of the impact absorber occurring against the passenger moderately adjustable.
However, in the afore-mentioned impact absorber, the concave rib is not formed in the first portion, and is formed in the second portion. For this reason, in a case where the first portion has a relatively large area surface, due to the lack of the concave rib (in the first portion) the impact absorber may not sufficiently serve as the impact absorber.
In other words, due to the impact from the outside of the vehicle (on the first portion) the impact absorber moves toward the passenger, and when colliding against the passenger via the trim it become deformed. During the deformation of the impact absorber, energy is consumed, and the mitigation of the impact on the passenger is thus obtained. However, when due to the impact from the outside of the vehicle the impact absorber is pressed to move toward the passenger, it may come in contact with other part(s) such as an impact beam and an outer pad which is disposed exteriorly of the impact absorber in the vehicle. In this case, when due to the specific shape of the other part local stress is given to the first portion of the impact absorber, the first portion is early deformed before the impact absorber collides against the passenger. As a result, the function of the impact absorber, the mitigation of the impact on the passenger cannot be securely attained.
In view of the above, it may be considered that the concave rib is also formed in the first portion of the impact absorber. In this case, the stress occurring when the impact absorber is subjected to impact may be a predetermined value or above, and a weak or fragile portion of the passenger such as a pubis may not be protected.